The present invention relates to a mold forming tool or pattern for the manufacture of casting molds from loose molding materials by one of the well known mold making processes in which the final compacting is achieved by pressing, the molds being used for casting of pieces presenting undercuts, especially annular objects such as slide- and/or counter-rings of axial face seals, the tool being elastically deformable in the region of the undercuts.
Molds for the production of cast pieces having undercuts can not as of the present be manufactured by simple compression molding processes. Such molds are therefore manufactured according to relatively complicated shaping procedures, such as, for example, the lost wax casting process, casting using lost patterns, or utilizing casting molds with inserted cores. Undercuts may also subsequently be ground out of rough castings.
For example, an axial face seal, at the upper rim of its outer circumference, includes, as an undercut, an annularly extending support collar for an elastic O-ring. The support collar is formed by grinding out a rough casting which has accordingly been cast to initially be larger than the desired finished size.
This manner of producing articles with undercuts is very laborious and expensive. Additional tools, to some extent expensive materials and time-consuming process steps are required, so that especially such articles can not be economically mass produced.
U.S. Pat. No. 3,570,585 discloses patterns for the production of casting molds made by a cold or heated process and provided with a back draft portion in the region where an undercut is to be produced. In this case, the pattern includes, in the region of the undercut to be formed, an elastic edge portion having a shape corresponding to the undercut when the portion is in its unstressed rest condition. During withdrawal of the pattern from the mold, the elastic portion is deformed by the resistance of the hardened mold material so that no destruction of the resulting back draft portion occurs. The deformation of the elastic portion by the action of the hardness of the mold material requires however a high degree of stability and hardness of the mold materials, so that this tool can be used to form back drafts only in hardened molding materials. Yet compared to the making of molds from loose molding materials wherein the final compacting is achieved by pressing, mold making using hardened molding materials is considered more laborious and expensive.